1. Field of the Invention
The disclosures herein generally relate to an optical beam scanner and a laser radar unit.
2. Description of the Related Art
An object type determining apparatus is known that uses a scanning laser radar apparatus installed in a vehicle to detect preceding vehicles and obstacles on the road and/or lane markers such as white lines and cat's eyes. The laser radar apparatus may detect a preceding vehicle or an obstacle that is ahead of the vehicle by irradiating laser light in a forward direction ahead of the vehicle and receiving the laser light reflected by the preceding vehicle or obstacle.
FIG. 1 is a block diagram showing an exemplary configuration of a scanning laser radar apparatus. The scanning laser radar apparatus shown in FIG. 1 includes a light transmitting unit 910, a light receiving unit 920, and an ECU (electronic control unit) 930. The light transmitting unit 910 and the light receiving unit 920 are arranged at the front side of the vehicle so that objects located ahead of the vehicle may be detected.
The light transmitting unit 910 includes a semiconductor laser diode (referred to as “LD” hereinafter) 911 that irradiates pulsed laser light, an optical scanner 912, an input optical system 913 that guides the light from the LD 911 to the optical scanner 912, and an output optical system 914 that controls the tilt angle from the road surface of a light beam that has passed the optical scanner 912, for example. The LD 911 is connected to the ECU 930 via a LD drive circuit 915 and is configured to irradiate laser light according to an LD drive signal from the ECU 930. The optical scanner 912 is connected to the ECU 930 via an optical scanner drive circuit 916 and is configured to repetitively scan the light beam irradiated from the LD 911 in the horizontal direction at a predetermined frequency based on a light scanning drive signal from the ECU 930. The scanning angle of the light beam irradiated from the optical scanner 912 is detected by a scanning angle monitor 917 and is output to the ECU 930 as a scanning angle signal. By supplying the scanning angle signal as feedback for the light scanning drive signal, the scanning angle and the scanning frequency may be controlled.
The light receiving unit 920 includes a light receiving lens 921 and a light receiving element 922. Laser light reflected by an object located ahead of the vehicle enters the light receiving element 922 via the light receiving lens 921 and a mirror element (not shown), for example. The light receiving element 922 may be a photodiode, for example, and is configured to output an electric signal with a voltage corresponding to the intensity of the reflected light entering the light receiving element 922. The electric signal output by the light receiving element 922 is amplified by an amplifier 941 and output to a comparator 942. The comparator 942 compares the output voltage of the electric signal from the amplifier 941 with a reference voltage V0 and outputs a predetermined light receiving signal to a time measuring circuit 943 when the output voltage is greater than the reference voltage V0.
The time measuring circuit 943 also receives the LD drive signal that is output to the LD drive circuit 915 from the ECU 930 and outputs to the ECU 930 as time measurement data the time it takes for the predetermined light receiving signal to be generated after the LD drive signal is output; i.e., time difference between the time point at which the laser light is irradiated and the time point at which the reflected light is received. Based on the time measurement data, the ECU 930 may calculate the distance of the object from the laser radar apparatus.
In the above scanning laser radar apparatus, the optical scanner 912 of the light transmitting unit 910 may include a polygon mirror or a galvano mirror, for example. FIG. 2 is a diagram showing an exemplary configuration of the optical scanner 912 shown in FIG. 1. In FIG. 2, the LD 911 and the input optical system 913, which may be a collimator lens, for example, are arranged at the side of a scanning mirror 951 such as a polygon mirror. In this example, laser light irradiated from the LD 911 passes through the input optical system 913 to be reflected by a mirror 952 and irradiated on a mirror surface 951a of the scanning mirror 951. The scanning mirror 951 rotates around a rotational axis 951b, and since light irradiated on the mirror surface 951a of the scanning mirror 951 is reflected by the mirror surface 951a, a laser beam may be scanned over a wide range in the horizontal direction. In this way, distance measurement over a wide range may be possible.
Presently, there is an ongoing demand for techniques related to two-dimensional scanning that involves scanning a light beam in the vertical direction as well as the horizontal direction and multiple-line scanning that involves horizontally scanning multiple light beams having different measurement ranges in the vertical direction. To realize such two-dimensional scanning or multiple-line scanning, a structure is known that serially connects scanning devices such as mirrors having scanning angles that vary by 90 degrees to scan a light beam in the vertical direction right after scanning a light beam in the horizontal direction. Also, a structure is known for facilitating multiple-line scanning by inclining the reflection surfaces of a rotating polygon mirror with respect to the optical axis and varying the inclining angles of the reflecting surfaces.
For example, Japanese Laid-Open Patent No. 9-274076 discloses a laser radar apparatus that uses a polygon mirror with varying plane-inclining angles. To reduce the difference in the vertical measuring ranges at the right and left ends of a measuring area, the disclosed apparatus has a laser diode arranged at the rear upper part of the polygon mirror. The disclosed apparatus also has a mirror arranged in front of the polygon mirror so that a laser beam irradiated from the laser diode may enter the mirror surface of the polygon mirror from the front side.
Japanese Laid-Open Patent No. 2009-98111 discloses a structure including deflecting units such as mirrors arranged under a light emitting element so that light may be irradiated 360 degrees in all directions.
However, the conventional multi-beam scanning method using a rotating polygon mirror with inclining planes may cause the vertical irradiation angle (also referred to as “vertical irradiation angle” or “vertical output angle”) to change as the horizontal scanning angle (also referred to as “horizontal irradiation angle” or “horizontal output angle”) widens. That is, when a light beam from a LD enters a mirror surface face-to-face, the light beam is reflected at a vertical output angle that is twice a predetermined inclining angle. However, as the polygon mirror rotates, the light beam may enter the mirror surface at a narrower angle with respect to the mirror surface. In this case, an adequate irradiation angle in the vertical direction cannot be secured. Considering the overall measuring area to be measured using multiple scanning beams, when the horizontal scanning range is relatively wide such that horizontal scanning may be performed over a total scanning angle exceeding 60 degrees, for example, the vertical measuring range may be limited as the horizontal scanning angle increases so that an adequate measuring range cannot be secured.
The laser radar apparatus disclosed in Japanese Laid-Open Patent No. 9-274076 cannot effectively prevent beam distortions when scanning is performed over a wide angle. Also, since the LD is arranged at the upper part of the polygon mirror, miniaturization of the unit, particularly, the reduction of the unit thickness may be difficult.
Further, in the above laser radar apparatus, light irradiated from the LD may be scanned but the light receiving unit is not scanned so that the detection range may be limited, and if attempts are made to widen the detection range, the detection sensitivity may be degraded.
The laser radar apparatus disclosed in Japanese Laid-Open Patent No. 2009-98111 has the light emitting element arranged above the deflection units such as mirrors so that the length of the apparatus may not be reduced and miniaturization of the apparatus may be difficult.
It is noted that measures may be taken to reduce the length of the apparatus by arranging the light beam to enter the mirror from the side in a horizontal direction with respect to the mirror. However, when the light beam is arranged to enter a rotating mirror from a horizontal direction, the scanning light beam may be distorted and the measuring area may be deformed into a trapezoid shape, for example, so that measurements cannot be made on the desired measuring area.